1. Field of the Invention
The present invention relates to a dispersion-compensating fiber, and a dispersion-compensating module and a hybrid optical fiber link using the same.
2. Description of the Related Art
Systems such as a long-distance nonreproducing relay system using an optical amplifier, such as an erbium-doped optical fiber amplifier, in a waveband of 1.53 xcexcm to 1.63 xcexcm are currently available on the commercial market. With the dramatic increase in communication capacity, the development of wavelength division multiplexing (WDM) transmission is proceeding rapidly.
In the attempt to achieve higher transmission speeds in this type of large-capacity WDM transmission, waveform deterioration caused by accumulated dispersion of the optical fiber (for sake of convenience, hereinafter termed xe2x80x9coptical fiber for transmissionxe2x80x9d), which the optical signal is transmitted on, becomes problematic. Accordingly, there has been proposed a dispersion management system using a hybrid optical fiber link which compensates positive accumulated dispersion, accumulated by transmission along the optical fiber for transmission, by using a dispersion-compensating fiber having negative chromatic dispersion with a comparatively large absolute value in a 1.55 xcexcm band. Accumulated dispersion is chromatic dispersion which accumulates when an optical signal is transmitted on an optical fiber.
Recently, there have been proposals for a chromatic dispersion slope-compensating type dispersion-compensating fiber, which can compensate chromatic dispersion and chromatic dispersion slope so as to improve the transmission characteristics in WDM transmission. For instance, Japanese Unexamined Patent Application, First Publication No. Hei 10-325913 and the like disclose a dispersion-compensating fiber which compensates accumulated dispersion in a case where the optical fiber for transmission comprises a single-mode optical fiber having zero dispersion in a band of 1.3 xcexcm (hereinafter abbreviated as SMF for 1.3 xcexcm), and Japanese Unexamined Patent Application, First Publication No. 2000-47048 and the like disclose a dispersion-compensating fiber which compensates accumulated dispersion in a case where the optical fiber for transmission comprises a non-zero dispersion-shifted fiber (NZ-DSF). The NZ-DSF has chromatic dispersion with a comparatively small absolute value in a band of 1.55 xcexcm, but the zero dispersion wavelength is deviated from 1.55 xcexcm in order to prevent four-wave mixing, which is one type of nonlinear effect.
An optical fiber having a zero dispersion wavelength close to 1.55 xcexcm, where quartz loss is at its minimum, is termed a dispersion-shifted fiber (hereinafter xe2x80x9cDSFxe2x80x9d) and is in practical use.
When there is zero dispersion wavelength close to 1.55 xcexcm as described above, four-wave mixing occurs and the transmission characteristics deteriorate. For this reason, the DSF has been deemed unsuitable for WDM transmissions close to the C-band (wavelength of 1.53 xcexcm to 1.565 xcexcm), which is widely used in conventional WDM transmission.
However, in view of the recent increase in communication capacity, the waveband used in such transmissions has been increased to what is termed an L-band (wavelength of 1.565 xcexcm to 1.625 xcexcm).
Since the DSF does not have a zero dispersion wavelength in the L-band, it is suitable for WDM transmission in the L-band, and, in combination with a dispersion-compensating fiber, it is possible to provide a large-capacity and high-speed hybrid optical fiber link which has excellent transmission characteristics. This has a considerable advantage that existing DSF can be used.
Accordingly, the present inventors considered how to provide a dispersion-compensating fiber which can compensate the chromatic dispersion of a DSF in the L-band, and, as a result of varied investigations, succeeded in perfecting the present invention.
The present inventors considered the characteristics appropriate to a dispersion-compensating fiber, as yet undeveloped, which would be capable of compensating accumulated dispersion of a DSF in the L-band, and a variety of design conditions, such as the refractive index profile, which are necessary to achieve those characteristics, and perfected the invention described below.
In order to achieve the above objects, the dispersion-compensating fiber according to the present invention comprises a central core segment; an intermediate core segment which is provided around the central core segment; a ring core segment which is provided around the intermediate core segment; and a clad which is provided around the ring core segment, wherein the refractive index profile of the dispersion-compensating fiber being such that the intermediate core segment has a lower refractive index than the central core segment, the ring core segment has a refractive index which is higher than that of the intermediate core segment and lower than that of the central core segment, and the clad has a refractive index which is lower than or equal to that of the ring core segment and higher than that of the intermediate core segment; and when the dispersion-compensating fiber is linked with a dispersion-shifted fiber which satisfies the following conditions: (A) single-mode transmission is actually achieved at a wavelength of 1.565 xcexcm (B) the zero dispersion wavelength is within the range of 1.55 xcexcmxc2x10.05 xcexcm (C) at all wavelengths of between 1.525 xcexcm and 1.575 xcexcm, chromatic dispersion is greater than or equal to xe2x88x923.5 ps/nm/km and less than or equal to +3.5 ps/nm/km (D) the dispersion slope at the zero chromatic dispersion is greater than or equal to +0.05 ps/nm2/km and less than or equal to +0.085 ps/nm2/km, the residual chromatic dispersion is below +1.5 ps/nm/km in all or part of the band of wavelengths from 1.565 xcexcm to 1.625 xcexcm.
A dispersion-compensating module according to the present invention uses the dispersion-compensating fiber described above.
A hybrid optical fiber link according to the present invention comprises the dispersion-compensating fiber described above; and a dispersion-shifted fiber which is linked with the dispersion-compensating fiber and which satisfies the following conditions (E) single-mode transmission is actually achieved at a wavelength of 1.565 xcexcm (F) the zero dispersion wavelength is within the range of 1.55 xcexcmxc2x10.05 xcexcm (G) at all wavelengths of between 1.525 xcexcm and 1.575 xcexcm, chromatic dispersion is greater than or equal to xe2x88x923.5 ps/nm/km and less than or equal to +3.5 ps/nm/km (H) the dispersion slope at the zero dispersion wavelength is greater than or equal to +0.05 ps/nm2/km and less than or equal to 0.085 ps/nm2/km.
Although different symbols (A) to (D) and (E) to (H) are used above, they represent identical contents, (A) to (D) corresponding to (E) to (H).
According to the present invention, chromatic dispersion of the DSF can be compensated in the L-band. Therefore, the hybrid optical fiber link can be provided which is suitable for WDM transmission and long-distance transmission using an existing DSF.